This invention relates generally to a novel ceramic cased capacitor and related methods of production. More particularly, this invention relates to a ceramic cased capacitor having a high temperature operating capability, together with a high lead pull strength and substantially improved resistance to moisture penetration to avoid moisture induced failures.
Ceramic dielectric capacitors in general are well known in the art for use in a wide range of electronic circuit applications, for example, for use as a charge storage device, a circuit coupling or decoupling device, a filtering device, etc. Such capacitors conventionally comprise at least two conductive plates encased in facing relation at a predetermined spacing or gap within a selected dielectric casing material, typically such as a ceramic based material formulated to have a selected dielectric constant. With this construction, the capacitor has a charge storing capacity which is a function of the overlapping or "active" plate surface areas, the thickness of the dielectric material defining the interplate gap (dielectric thickness), and the dielectric constant of the casing material within the gap. In many electronic circuit applications, particularly in aerospace operating environments, it is desirable to provide a highly compact capacitor construction with a relatively high capacitance. In this regard, multiple layer capacitors have been developed with two groups of conductive plates of opposite polarity arranged alternately in a stack and encased within the selected dielectric casing material, with the multiple plates providing a significantly increased active plate surface area with a relatively small overall increase in capacitor size. See, for example, U.S. Pat. Nos. 3,235,939 and 3,456,313.
In the past, ceramic dielectric capacitors have been produced by formulating the selected dielectric material such as a barium titanate or the like into relatively thin sheets. While in a relatively flexible "green" state before firing, the ceramic sheets are electroded or silk-screened with a refractory metal to define thin conductive plates of selected area. A plurality of these ceramic sheets with conductive plates thereon are laminated into a stack and then fired to form the sheet into a rigid and dense, substantially monolithic casing structure having the conductive plates encased therein at a predetermined dielectric thickness. The conductive plates are shaped to provide thin edges exposed at the exterior of the casing for connection to appropriate conductive leads as by soldering or the like. In multiple plate capacitors, groups of the plate edges are appropriately coupled to each other by conductive metallization strips applied to the exterior of the casing, with the conductive leads being fastened to the metallization strips by soldering or the like.
A variety of problems and disadvantages have been encountered in the use of ceramic dielectric or monolithic capacitors of the general type described above. More specifically, the externally exposed edges of the encased conductive plates define entry sites along so-called knit line defects for ingress of moisture between the conductive plates and adjacent dielectric material. Such moisture ingress contributes to internal dendritic growth or increased ionic mobility which causes corresponding reductions in insulation resistance between adjacent conductive plates, which can lead to parametric or catastrophic failure. Moreover, thermal cycles encountered during normal capacitor operation can cause repeated vaporization and recondensing of minute moistures quantities, resulting in cracking and failure of the ceramic dielectric material. Still further, in a conventional ceramic capacitor, the externally formed solder joints used to connect the conductive leads sometimes provide inadequate mechanical lead pull strength especially at elevated operating temperatures. The external solder joints are also esthetically unacceptable and/or require insulation coverings for some circuit applications.
In the past, the problems of moisture penetration have been addressed primarily by encapsulating the entire monolithic casing within a jacket or coating of a suitable insulating material, such as an epoxy, potting compound, polymeric coating, etc. This jacket or coating is intended to seal the casing against moisture penetration, while simultaneously covering solder joints to enhance the appearance of the capacitor. Advantageously, the outer jacket additionally provides mechanical support for the conductive leads to increase the lead pull strength. However, the outer insulation jacket inherently increases the overall size and shape of the resultant capacitor, resulting in a failure to optimize the capacitance per unit volume, sometimes referred to as volumetric efficiency. Moreover, while the jacket provides some resistance to moisture penetration, moisture ingress may nevertheless occur by penetration between the jacket and the conductive leads, or by gradual migration directly through the jacket in accordance with the bulk permeability of the jacket material. Alternately, different thermal coefficients of expansion for the jacket and casing can result in moisture laden air being sucked in by the jacket when significant thermal cycles are encountered. When such moisture penetration occurs, delamination of the jacket from the ceramic casing often results in reduced insulation resistance, or short circuiting high voltage corona between the casing and the jacket. The differential thermal coefficients can also cause cracking failure of the ceramic casing as the jacket and casing expand and contract at different rates in response to high temperatures or thermal cycling operation. Attempts to minimize risk of such cracking failure have focused upon the use of resilient jacket materials which, unfortunately, do not provide acceptable mechanical support for the conductive leads.
Encapsulated monolithic casing capacitors are also limited to environments of use wherein outgassing of solvents and other volatiles can be tolerated. That is, encapsulating materials exhibit outgassing characteristics in varying degrees, wherein a gradual mass loss over a period of time is encountered as solvents and/or other condensable volatiles are outgassed to the surrounding environment. Such outgassing is unacceptable in some capacitor applications, such as in space and other sensitive environments.
There exists, therefore, a significant need for improvements in ceramic dielectric capacitors, particularly with respect to providing enhanced resistance to moisture penetration without the use of a conventional encapsulating outer jacket. Moreover, there exists a need for such improvements in a capacitor designed for substantially optimized volumetric efficiency and relatively high lead pull strength. The present invention fulfills these needs and provides further related advantages.